Kinetics of Calcium Zincate Formation

The kinetics of calcium zincate formation by the reaction Ca OH 2 s + 2 ZnO s + 4 H 2 O E ' lte → Ca OH 2 · 2 Zn OH 2 · 2 H 2 O s in KOH solutions were studied by a potential measuring technique at room temperature and by a volumetric technique at 10°, 19°, and 30°C. In the potential measuring technique, the progress of the reaction was monitored by measuring the potential difference between a zinc strip dipped in the electrolyte solution and a HgO / Hg reference electrode. This technique could be used to determine only the average rate of formation of calcium zincate. The rate of formation was not controlled by the dissolution and transport of ZnO , and also not by the transport of water in the medium. In the volumetric technique, the progress of the reaction was monitored by measuring the rate of decrease of the molar volume of the reactants during the calcium zincate formation. The data of the volumetric technique, in agreement with those of the potential measuring technique, indicated that the initial rate of formation of calcium zincate was controlled by the rate of formation of calcium zincate crystal nuclei and their growth in size. The observed S‐type curve of the volume change with time characterizes an initial slow rate (nuclei formation), an intermediate fast rate (nuclei growth), and then a final slow rate (depletion of reactants) which tapers asymptotically to zero. The data can be described in terms of an irreversible bimolecular‐type second‐order reaction in which Ca ( OH ) 2 and the product calcium zincate are treated as reactants. The rate expression is P = R 0 + P 0 1 + R 0 P 0 e − R 0 + P 0 kt where k was found to be 0.031 liter, per minute, per g mol. P 0 and R 0 refer to initial concentrations of Ca ( OH ) 2 and calcium zincate.